Metal detection systems are useful in detecting the unauthorized removal of metal items from a protected area as well as detecting metal objects that may be brought into a protected area. Particularly in retail environments, metal detection systems save stores hundreds of thousands of dollars by preventing the unauthorized removal of unpaid-for items from the store. In places such as schools, airports, and stadiums, metal detectors serve the purpose of preventing patrons from bringing in weapons or items that could cause harm to others or leaving with items that they have not purchased.
Metal detection systems are often combined with electronic article surveillance (“EAS”) systems. EAS systems are commonly used in retail stores and other settings to prevent the unauthorized removal of goods from a protected area. Typically, such a system is configured at an exit from the protected area. The system includes one or more transmitters, receivers and antennas, stored in a housing (such as an EAS pedestal) capable of generating an electromagnetic field across the exit, known as the “interrogation zone” or “detection region”. Articles to be protected are tagged with an EAS marker that, when active, generates a response signal when passed through this interrogation zone. An antenna and receiver in the same or another “pedestal” detects this response signal and generates an alarm and/or sends an alert message to monitoring personnel. Combination EAS/metal detection systems utilize the pedestals to detect both the unauthorized removal of goods as well as metal objects entering or exiting the interrogation zone.
One reason for combining metal detection functions in an EAS system is due to the problems EAS systems experience relating to their inability to detect unauthorized removal of tagged items when the tags are placed in a shielded environment, such as when EAS tags are contained in a metal-lined bag. Often, unscrupulous shoppers bring metal-lined bags to a store with the idea of placing an item having an EAS tag in the bag, and attempt to walk out of the store undetected. EAS detection systems that do not include metal detection capability may be defeated by utilizing this method. Therefore, in order to prevent this from occurring, EAS systems employ metal detection capabilities. With the emergence of metal detection integrated with EAS technology, EAS systems have become more and more robust and seamless in offering customers a complete solution to their loss prevention needs. The new “combination” system utilizes the existing EAS pedestals, thus maximizes efficiency in terms of cost, space and overall aesthetics of the system.
However, metal detection systems, whether they are stand-alone systems, or combination EAS/metal detection systems are not without their inherent problems. A problem that arises is when a metal detection or combination EAS/metal detection system systems has been installed in close proximity with some type large metal object, such as a metal door frame. Metal doors, such as, for example, sliding metal doors, are common in many retail store environments. These sliding metal doors tend to degrade the performance of metal detection. This is because when installed nearby a metal door frame, the electromagnetic field gradient, which is the essence of metal detection, is compromised as the door is in motion, leading to false alarms. Also, the amount of metal in the door as compared with the amount of metal present in a transitory metallic object such as a tag shield, e.g., foil-lined bag, results in a metal detection response signal from the door that is many orders of magnitude stronger than the response from the transitory metallic object such as the metal tag shield. While these metal doors have no effect on EAS detection, the interference with the metal detection capabilities of the system caused by the opening and closing of these doors and the amount of metal in the door can be quite severe.
Other attempts to lessen or eliminate the effects of metal doors in a metal detection zone have proven to be in adequate. Some of these include shielding and electrically separating the door and the pedestals, recommending a “safe” distance; and developing an entirely new stand-alone metal detection system that works independently from the EAS system. Since a sliding metal door has profound effects on the metal detection system, shielding the door from the antennas is not a practical option. The metal detection portion of combination EAS/metal detection systems are extremely sensitive to any changes in field gradient and therefore no reasonable amount of metal shield is adequate to prevent change in field gradient when the metal sliding doors are in motion. Placing the system at a “safe” distance, or utilizing non-EAS independent metal detection systems are not efficient as the methods defeat the purpose of integrating metal detection into EAS systems, which are more often than not installed in close proximity with sliding metal doors.
Moreover, metal detection systems typically lose metal detection accuracy over time due to drift or changes over time. Drift may be caused by numerous factors such as component movement and metal detection zone changes. For example, a signal received by the metal detection system may vary over time or drift even though the metal objects the system was designed to detect are not present, e.g., signal amplitude increases over time even though the metal objects are not present. The drift, e.g., increase in signal amplitude, may cause false metal detection alarms in which the number of false alarms may continually increase as drift becomes worse such that the system may end up constantly triggering false alarms. In order to fix the false alarm problem, metal detection system users such as retail stores may end up having to install a new metal detection system or may need a technician to manually recalibrate the system. Such actions to fix the false alarm problem may incur substantial cost and may even deter retail stores from fixing the problem, i.e., may simply turn off the metal detection system, leaving the store vulnerable to theft.
Therefore, what is needed is a system and method for nullifying the effects of moving metal doors in a metal detection interrogation zone taking into account environmental factors such as pattern drift.